In general, a WDM network (Wavelength Division Multiplexed-Optical Network) has become recognized as an ultimate network capable of coping with explosive network traffic increase due to advantages capable of providing bandwidth catering to recently explosively increasing bandwidth demand.
FIG. 1 is a schematic block diagram illustrating a general WDM-PON system.
A broadband light outputted from a BLS (Broadband Light Source, 100 is spectrum-divided (filtered) after passing a demultiplexing AWB (Arrayed Waveguide Grating, 120) by passing through an optical circulator, 110). The spectrum-divided light is injected into an optical transmitter (Tx, 130). In general, the optical transmitter (130) of the WDM-PON system includes an F-P LD or an RSOA (Reflective Semiconductor Optical Amplifier). The F-P LD or the RSOA amplifies an injected light and simultaneously reduces noise, and modulates to a transmitted signal. Lights of mutually different wavelengths outputted from each channel are inputted to the AWG (120), where the AWG (120) multiplexes the lights. The multiplexed signals are transmitted through an optical fiber.
FIGS. 2A to 2E are exemplary views illustrating device characteristics used for optical transmitter in a WDM-PON system according to prior art. When a light is injected into an F-P LD/RSOA (131), a gain saturation phenomenon is generated where a gain is reduced in reverse proportionate to gain as intensity of light increases, as shown in FIG. 2A. Noise of injected light can be inhibited when using the gain saturation phenomenon.
FIG. 2B illustrates an RIN (Relative Intensity Noise) in a frequency region, and it can be noted that when a light having an RIN of −110 dB/Hz and a power of −10 dBm is injected, noise is reduced by approximately 10 dB (A→B). That is, it can be confirmed that the noise is inhibited by approximately 1/10.
FIG. 2C illustrates an optical spectrum of F-P LD, and FIG. 2D illustrates an optical spectrum of RSOA. The F-P LD is a very general LD (Laser Diode) formed by a cavity formed by mirrors at both ends and a gain medium therebetween, and coated at the entire surface with anti-reflection coating (reflectance less than 0.1%(in order to inject the light. When the F-P LD is used for the optical transmitter (130), it can be noted as shown in FIG. 2C that a plurality of modes is formed by the cavity. Thus, the RSOA may be an F-P LD having a very low total reflectance (approximately 0.001%). As a result, as illustrated in FIG. 2D, it can be learned that the RSOA has almost no mode characteristic over F-P LD.
Meantime, the physical properties of optical transmitter (130) vary depending on relationship between wavelength (λINJ) of light injected from outside and wavelength (λF-P) of mode formed by cavity of F-P LD. FIG. 2E illustrates a result of degree of noise inhibited by detuning (λINJ-λF-P). Furthermore, an output light of F-P LD passes the AWG (120), where a frequency component is filtered (filtering effect) by pass characteristic of the AWG (120). At this time, an output spectrum of the F-P LD is changed by the detuning, whereby a frequency component filtered by the AWG (120) is resultantly changed to disadvantageously have an effect on the physical properties of the optical transmitter (130).
As noted above, as noise of the received optical signal is changed in response to the detuning, the physical properties deteriorate due to changed detuning when external environments such as temperature are changed even if operated under an optimal condition. Thus, power of injected light must be increased for color-free operation and to prevent the deteriorated physical properties. As a result, a high output powered BLS (100) must be used, which disadvantageously increases a power consumption of entire system to increase the price.
Meanwhile, although almost no influence is received by detuning when an RSOA is used, the RSOA lags behind in price competitiveness over the F-P LD to inevitably use a TEC, resulting in increased power consumption to a great extent. Korea Patent No.: 680918 (Feb. 2, 2007) is disclosed to provide a method for solving the detuning problem.
The Korea Patent No.: 680918 teaches three electrodes for injecting a current in the conventional F-P LD configuration, which are called triple contact F-P LD. The triple contact F-P LD can change a ratio of currents injected to each electrode to change wavelengths of a plurality of modes that oscillates by constantly maintaining an output power.
FIG. 3A is a schematic view illustrating a configuration of triple contact F-P LD according to prior art, and FIG. 3B is a schematic view illustrating a position of oscillation mode in response to a current injected to each electrode of the triple contact F-P LD in FIG. 3A, where I1 is a current injected to a first electrode (310), I2 denotes a current injected to a second electrode (320), and I3 denotes a current injected to a third electrode (330).
However, the triple contact F-P LD according to the prior art cannot satisfy a scope of wavelength keeping capable of coping with a broad temperature change of an actual field when a change rate (0.1 nm/° C.) in response to the temperature is considered.
Meantime, when a coherent light and incoherent light are injected from outside, and when oscillation wave of F-P LD and injection wave are matched, the F-P LD that has oscillated in multiple modes oscillates in a single mode at the injected wavelength. At this time, when the external temperature is changed to change the oscillation wavelength of the laser, laser characteristics are changed, and the F-P LD returns to multiple mode, and becomes difficult to be used for WDM. Thus, the laser can track the transmission characteristics, only when oscillation wavelength is changed to keep oscillating at an originally oscillated wavelength and to allow tracking the oscillation wavelength. This requirement is particularly important when the oscillation wavelength spacing of F-P LD is broader than line width of injected light. Thus, the wavelength tracking is essential to realize the colorless characteristic by applying the F-P LD to super high speed WDM system.
When a modulator-integrated reflective semiconductor optical amplifier is manufactured, a colorless light source capable of high speed modulation free from wavelength tracking can be realized. However, the reflective semiconductor optical amplifier must be attached with a TEC because the reflective semiconductor optical amplifier is narrow in operating wavelength scope and high in operating current. Although this may be thought as one of subclasses of wavelength tracking F-P LD, it is difficult to remove the TEC.
Meantime, the current optical communication system uses various modulation methods {QPSK (Quadrature Phase Shift Keying), DQPSK (Differential Quadrature Phase Shift Keying, QAM (Quadrature Amplitude Modulation)} capable of simultaneously performing the phase modulation and intensity modulation. However, an external modulator used for this purpose is disadvantageously expensive, and low in competitiveness in terms of maintenance/repair. Thus, it is imperative that a light source capable of accommodating various modulation methods be developed while operable in colorless operation.